Antenna feed networks are commonly employed in RF systems that operate in various microwave or millimeter wave frequency bands such as automotive radar, according to one example. Typical antenna feed networks include power splitters, directional couplers, rat-race hybrids, branch-line couplers, etc., that usually require the isolated ports terminated with a resistance about equal to the characteristic impedance of the transmission lines. Generally, an ideal termination circuit is a one-port device that absorbs RF energy incident on the port and reflects none. In general, it is typically sufficient that the termination is effective over the frequency band of operation.
At higher microwave frequencies, terminations are often achieved by dispensing liquid resistive material over an area on open transmission lines, such as microstrip, that attenuates RF energy. However, this technique is difficult to implement in shielded transmission line structures, such as stripline, where the conducting strip is sandwiched between dielectric substrates, with ground metallization on the outer sides.